(CNN) -- U.S. Senate races in Kansas and Missouri and a ballot measure seen as a referendum on President Obama's health care reform are among Tuesday's primary elections.

Missouri's Proposition C is a test of support for part of the new federal health care law, as voters decide if Missouri residents should be allowed to opt out of mandatory health insurance.

Early returns show the measure succeeding handily, with more than 70 percent voting to allow residents to opt out. Mason-Dixon pollster Brad Coker told the St. Louis Post-Dispatch he expected the proposition to pass because of an anticipated heavier Republican turnout. But he also expects a court challenge if the measure passes. Some states have already passed similar legislation, but this will be the first test at the ballot box.

Elsewhere in Missouri, there shouldn't be as much drama in the fight is to succeed retiring Republican Sen. Kit Bond.

Missouri Secretary of State Robin Carnahan, who hails from one of the state's leading political dynasties, is expected to easily win the Democratic primary. Rep. Roy Blunt, the former House Majority Whip, is expected to win in a nine-candidate Republican field. His only competition appears to be coming from Chuck Purgason, who enjoys the backing of many Tea Party activists.

The GOP Senate battle is the marquee race in Kansas, as two congressmen fight to replace fellow Republican Sen. Sam Brownback, who is running for governor rather than for re-election.

Rep. Jerry Moran leads Rep. Todd Tiahrt and two other challengers in polls.

Baker University administrator Lisa Johnston leads the five Democratic candidates on the ballot in early returns, but whoever captures the GOP contest will be considered the overwhelming favorite in the general election.

"The winner of the Republican primary essentially becomes the winner in November because Kansas is a state where Republicans dominate in federal races," said Nathan Gonzales, political editor for the nonpartisan Rothenberg Report.

Brownback, who made a bid for the White House in 2008, won the GOP gubernatorial nomination and will face State Sen. Tom Holland, who had no opposition in the Democratic primary, in November.

In Michigan, the state with the second highest unemployment level in the country, the contest for the GOP gubernatorial nomination is the headliner. The winner will start the general election with an advantage.

Recent state surveys indicate that businessman and former Gateway CEO Rick Snyder, Rep. Peter Hoekstra, the senior Republican on the House Intelligence Committee, and Attorney General Mike Cox are in a three-way battle, with Oakland County Sheriff Mike Bouchard in fourth place in a five-candidate field.

State Rep. Andy Dillon and Lansing Mayor Virg Bernero are facing off for the Democratic nomination. Two-term Democratic Gov. Jennifer Granholm is term-limited.

Michigan's economy continues to sputter and with a sitting Democratic president and an outgoing Democratic governor, "it looks like Michiganders might be willing to let Republicans have a shot at governing," said Gonzales.

The GOP is hoping to score a pickup in November in Michigan's 1st Congressional District, where longtime Democratic Rep. Bart Stupak announced in April that he would not run for re-election after being battered for changing his no vote on health care to a key yes vote.